With a rapid progress of LED techniques, especially, techniques for a blue LED (light-emitting diode) which is a III nitride semiconductor light-emitting element, a white LED (hereinafter referred to as “light-emitting element”) which is a combination of a blue LED and a fluorescent substance (e.g. yellow fluorescent substance) has been put into practical use. The white LED has a luminous efficiency (efficiency of conversion of electricity to light) higher than an incandescent lamp, and is expected to become, in the future, an energy saving light source having a luminous efficiency higher than a fluorescent lamp.
It is possible to realize a planar light source by mounting a plurality of light-emitting elements on a substrate. The planar light source can be used as an illumination light source or a backlight for a non-light-emitting display device such as a liquid crystal display device. Japanese Patent Application Publication Tokukai No. 2007-227389 (published on Sep. 6, 2007, and hereinafter referred to as Patent Document 1) discloses a liquid crystal display device including a backlight in which blue, green and red LEDs are provided on a back side of a liquid crystal display panel. A method thus using three-color LEDs has the advantage of widening a color reproducibility region of a liquid crystal display device, whereas has the disadvantage of a necessity of increase in the distance between the backlight and the liquid crystal display panel so that a good mixture of the three colors is realized in the liquid crystal display panel. In contrast, when a white LED (light-emitting element) composed of a blue LED and a fluorescent substance is used as a backlight, it is possible to realize a good color mixture even if the backlight is provided to be close to the liquid crystal display panel. Such a flat display device can be suitably used as a wall-hung display and a portable display. However, when the backlight is provided to be close to the liquid crystal display panel, the influence due to production tolerance of the light-emitting elements themselves comes out. This causes in-plane distribution of chromaticity to become visible.
Japanese Patent Application Publication Tokukai No. 2007-80530 (published on Mar. 29, 2007, and hereinafter referred to as Patent Document 2) discloses an LED illumination device using a white LED, in which a color mixture is substantially the same as a target chromaticity coordinate e of white. For example, according to the LED illumination device disclosed in Patent Document 2, (i) a group of white LEDs in which x and y of chromaticity coordinates (x, y) are larger than the target chromaticity coordinates e and (ii) a group of white LEDs in which x and y of the chromaticity coordinates (x, y) are smaller than the target chromaticity coordinates e are combined so that a mixed color has the chromaticity close to the target chromaticity coordinates e.
However, the LED illumination device disclosed in Patent Document 2 still has the following problems to be solved.
First, in a planar light source in which light-emitting elements are provided two-dimensionally, unevenness of color and unevenness of luminous intensity must be fully restrained.
Second, when the planar light source in which the light-emitting elements are provided two-dimensionally is used as a backlight of a liquid crystal display device or the like, unevenness of color and unevenness of luminous intensity of the light-emitting elements must be fully restrained in a display panel provided near a backlight. In other words, a flat display device must be realized in which the backlight can be provided close to the display panel.
Third, a method which is directed to a combination of light-emitting elements and which is suitable for reducing in-plane distribution must be provided.
Fourth, a displaying method suitable for a display device using a planar light source whose in-plane distribution is reduced must be provided.
Fifth, the utilization ratio (yield) of light-emitting elements used in a planar light source must be improved.